Over the past 50 years, the field of organic photochemistry has produced a wealth of information, from reaction mechanisms to useful methodology for synthetic transformations. Many technological innovations have been realized during this time due to the exploits of this knowledge, including photoresists and lithography for the production of integrated circuits, photocharge generation for xerography, multidimensional fluorescence imaging, photodynamic therapy for cancer treatment, photoinitiated polymerization, and UV protection of plastics and humans through the development of UV absorbing compounds and sunscreens, to name a few.
The scientific basis of many of these processes continues to be utilized today, particularly in the development of organic three-dimensional optical data storage media and processes.
It is known that fluorescent properties of certain fluorophores may be changed (quenched) upon protonation by photogeneration of acid Two-photon induced photoacid generation using onium salts and short pulsed near-IR lasers in the presence of a polymerizable medium has been reported, resulting in two-photon photoinitiated cationic polymerizations The inherent three-dimensional features associated with two-photon absorption provides an intriguing basis upon which to combine spatially-resolved, two-photon induced photoacid generation and fluorescence quenching with two-photon fluorescence imaging
The quadratic, or nonlinear, dependence of two-photon absorption on the intensity of the incident light has substantial implications (dw/dt oc I2). For example, in a medium containing one-photon absorbing chromophores significant absorption occurs all along the path of a focused beam of suitable wavelength light. This can lead to out-of focus excitation. In a two-photon process, negligible absorption occurs except in the immediate vicinity of the focal volume of a light beam of appropriate energy. This allows spatial resolution about the beam axis as well as radially, which circumvents out-of-focus absorption and is the principle reason for two-photon fluorescence imaging. Particular molecules can undergo upconverted fluorescence through nonresonant two-photon absorption using near-IR radiation, resulting in an energy emission greater than that of the individual photons involved (upconversion). The use of a longer wavelength excitation source for fluorescence emission affords advantages not feasible using conventional UV or visible fluoresence techniques, e.g., deeper penetration of the excitation beam and reduction of photobleaching, and is particularly well-suited for fluorescence detection in multilayer coatings.
Rentzepis et al. reported two-photon induced photochromism of spiropyran derivatives at 1064 nm. Analogous to single-photon absorption facilitated isomerizion, the spiropyran underwent ring-opening isomerizion to the zwitterionic colored merocyanine isomer. The merocyanine isomer underwent two-photon absorption at 1064 nm, resulting in upconverted fluoresence. However, spiropyrans are known to undergo photobleaching and photodegradation upon prolonged exposure, and hence are not suitable for long-term use. Nonetheless, an intriguing model for 3-D optical storage memory was proposed. An intriguing bacteriorhodopsin-based holographic recording media and process, using two-photon excitation, has been reported by Birge et al.
The synthesis and characterization of organic fluorescent dyes with high two-photon absorptivity has been reported. Several of these dyes also undergo substantial changes in the absorption and fluorescence spectral properties in the presence of strong acid, i.e., they undergo protonation, affording changes in their polarizability, absorption and emission maxima and fluorescence quantum yields.
With the ever-pressing demand for higher storage densities, researchers are pursuing a number of strategies to develop three-dimensional capabilities for optical data storage in organic-based systems. Among the various strategies reported are holographic data storage using photopolymerizable media photorefractive polymers, and two-photon induced photochromism, to mention a few.
In light of the foregoing, there is a need for an increased density of data storage, particularly for CD/DVD systems.